Polybenzimidazole (PBI), or poly-2,2′(m-phenylene)-5,5′-bibenzimidazole, is a polymer that is resistant to strong acids, bases, and high temperatures (e.g., up to 500° C.). It has a heat resistance temperature above 430° C. PBI also exhibits excellent, mechanical strength, wear resistance and chemical resistance properties. Therefore, PBI can be used for applications that operate under extremely high temperature, mechanical loads and chemical corrosive environments.
PBI has been used over the past years to form membranes, electrically conductive materials, fire resistant materials, ultrafilters, and other types of separatory media. For example, fibers may be formed from PBI (i.e., PBI fibers) and woven into fabrics that are used in high temperature fire resistant suits, which are able to withstand temperatures of up to 600° C. Thin films or flat sheets of PBI are typically swollen with other solvents, such as phosphoric acid, for stability and electrical conductivity. Solid forms (e.g., rods and blocks) of PBI may also be formed by heat compression molding of a powdered PBI resin.
However, the poor film-forming characteristics of PBI have prevented use of PBI in film or coating applications. PBI has very poor solubility in conventional organic solvents, but may be dissolved in highly polar, aprotic organic solvents, such as dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAc), N,N-dimethylforamide (DMF), or N-methylpyrrolidinone (NMP), to form a PBI solution. PBI's molecular structure is dependent upon hydrogen bonding by its imidazole groups. Disrupting the hydrogen bonding causes crazing and cracking that is especially pronounced when forming thin, flat articles, such as films. Therefore, additives (i.e., acidic compounds or lithium salts) may be introduced to the PBI solution to stabilize the hydrogen bonding in the polymer matrix.
Most commonly, PBI is stabilized using acidic compounds, such as sulfuric acid, phosphoric acid and organic derivatives thereof. Such acidic compounds do not result in substitution of PBI with sulfate or phosphate, but instead result in protonation of a polymer backbone of PBI. While not wishing to be bound by any particular theory, it is postulated that the acidic compounds penetrate the PBI matrix, breaking up the crystal ordering. Thus, combining PBI with the acidic compounds tends to “plasticize” PBI, resulting in formation of PBI gels, which are often referred to as “dopes.” The PBI gels or dopes are used in constructing PBI hydrogen fuel cells (or similar devices) where integration of water into the matrix is important for proton transport. However, a PBI gel or dope is not useful for solution casting of dry, free-standing films, especially when the resulting films will be exposed to moisture in their applications. In particular, the additives in the PBI solutions are known to cause deleterious effects, such as swelling, in PBI films when exposed to water or moisture. When the additives are not used, solution-cast PBI films tend to stress crack and fissure, especially upon exposure to increased temperatures (e.g., temperatures greater than 150° C.) due to loss of the high boiling solvent. Such difficulties have prevented the formation of free-standing films of PBI.
As an alternative to forming PBI solutions including acidic compounds, PBI may be modified at the molecular level. PBI includes imidazole groups with reactive nitrogen atoms (i.e., imidazole nitrogen atoms) that may be used for molecular substitution (i.e., grafting) or for forming new PBI polymers using aldehyde and amine precursors (monomers) prior to polymerization. Altering monomers of PBI before polymerization is difficult, and the molecular morphology of the resulting polymer may be considerably different from that of homogeneous PBI. Synthetic N-substitution of PBI after polymerization has been performed with varying success. However, additional synthetic acts and work-up procedures are required to form N-substituted PBI compounds. Furthermore, the hydrogen bonding within the polymer matrix is disrupted due to the N-substitution at the imidazole nitrogen atoms. The additives are often used to stabilize the polymer matrix of N-substituted PBI compounds, thus, complicating formation of stable free-standing films from PBI.
Polymer compositions that include PBI crosslinked and/or blended with other polymers have also been used for PBI film formation. Although blending other polymers with PBI may provide some stabilization of the polymer matrix, the majority of such polymer compositions are gels or dopes that are used for fibers or fuel cells. For example, a blend of PBI and ULTEM® 1000 polyetherimide in DMAc has been used to form fibers. However, thin films formed from such a blend phase separate, turn opaque and/or fracture without the addition of the previously described additives, especially in the presence of moisture.